Add FetchSitesPayload to newFetchSitesAction

[ashiagr]

This PR adds an empty FetchSitesPayload to newFetchSitesAction in the Login library as part of the Jetpack app filters support to fetch sites API.

Related Issue: wordpress-mobile/WordPress-Android#14299 Related PRs: FluxC: https://github.com/wordpress-mobile/WordPress-FluxC-Android/pull/1936 WPAndroid: https://github.com/wordpress-mobile/WordPress-Android/pull/14397 WCAndroid: https://github.com/woocommerce/woocommerce-android/pull/3805

Merge Instructions

• Make sure https://github.com/wordpress-mobile/WordPress-FluxC-Android/pull/1936 is merged to FluxC develop and tag is created for it.
• Update FluxC tag in build.gradle here in this PR.
• Make sure all checks are passed.
• Merge this PR with develop.